1. Field of the Invention
The present invention relates generally to data communications and, more specifically, the present invention relates to low voltage integrated circuits transmitting data over higher voltage communications lines.
2. Background Information
As the speeds of electronic circuits increase, there is a continuing trend to reduce the operating voltage of the integrated circuits from the traditional 5 volt DC power supply. For instance, present day integrated circuits commonly operate at approximately 3.3 volts and 2.2 volts. Lower voltage integrated circuits will soon be desired to accommodate higher integrated circuit speeds in the near future. However, many existing communications systems require 5 volt peak-to-peak signals that cannot be easily produced by these lower voltage integrated circuits.
At the present time, there are some known methods of producing higher voltage data communications signals, such as for example 5 volt peak-to-peak signals, with lower voltage integrated circuits, such as for example approximately 3.3 volts or lower. However, most known methods suffer from a variety of problems including undue amounts of noise and interference, and/or are impractical to implement.
FIG. 1A is a schematic of one known method of producing higher voltage level data communications signals with lower voltage integrated circuits. A data communications circuit is illustrated with an isolation transformer 101 including a center-tap 103 coupling primary winding 105 to Vcc. Primary load resistors 109 and 111 are coupled between center-tap 103 and the respective ends of primary winding 105 of transistor 101. A data communications line 121 is coupled to the ends of secondary winding 107. As shown in FIG. 1B, the characteristic impedance Z0 of data communications line 121 may be alternatively represented as a secondary load resistor 115, which would be coupled in parallel across the ends of secondary winding 107. Two current sinking drivers 117 and 119 are connected to the ends of primary winding 105.
One problem with the data communications circuit illustrated in FIG. 1A is that center tap 103 results in inherent second harmonic distortion, which manifests as common mode signal currents and attendant electromagnetic interference (EMI) problems.
With the dual current sinking drivers 117 and 119 at the transformer ends of primary winding 105, and with Vcc applied to center-tap 103, the circuit of FIG. 1A resembles a push-pull audio power amplifier in topology and current sinking drivers 117 and 119 only drive half of the primary winding 105 at a time. Since the remaining non-driven half of the primary winding 105 is rather tightly magnetically coupled, autotransformer action occurs from the non-driven winding. Consequently, all of the parasitic reactances and IR drops associated with the non-driven winding, as well as the reflected non-linear B-H magnetizing characteristic from the transistor core appear as additional loads to the driving side of the circuit. Since there is no negative feedback in the circuit, as would be used in an analogous audio power amplifier to reduce distortion, the circuit of FIG. 1A suffers from quite high harmonic distortion due to non-linear loading. At frequencies of operation occurring in networking systems, this translates into excessive EMI and loss of signal quality.
FIG. 2 is a schematic of another known center-tapped primary data communications circuit similar to the circuit shown in FIG. 1A. In particular, FIG. 2 shows an isolation transformer 201 with a center-tap 203 on primary winding 205. Data communications line 221 is coupled across the ends of secondary winding 207. A characteristic impedance of data communications line 221 is represented in FIG. 2 as a secondary load resistor 215, coupled in parallel across the ends of secondary winding 207. Two current sinking drivers 217 and 219 are connected to the transformer ends of primary winding 205. Each end of primary winding 205 is coupled to center-tap 203 through primary load resistors 209 and 211, respectively.
In order to achieve acceptable power consumption, transformer 201 of FIG. 2 is also driven in a push-pull fashion, similar to the circuit discussed above in FIG. 1A. Therefore, since the non-driven half of the primary winding 205 is rather tightly magnetically coupled, autotransformer action occurs in the non-driven half of primary winding 205, and all of its parasitic reactances and IR drops, as well as its reflected non-linear B-H magnetizing characteristic from the transformer core, appear as additional loads to the driving side of the circuit resulting in high harmonic distortion, excessive EMI and loss of signal quality.
FIG. 3 is a schematic of a known data communications circuit utilizing a step-up transformer 301. Principal problems associated with step-up transformer 301 include the increased circuit sensitivity due to step-up transformer 301 parameters and the very low impedances that result on the primary winding side of step-up transformer 301. In particular, primary winding 305 of transformer 301 is driven end-to-end by current generator 317. Primary load resistor 309 is coupled in parallel across the ends of primary winding 305. Data communications line 321 is coupled end-to-end across secondary winding 307. The characteristic impedance of data communications line 321 is represented in FIG. 3 as a secondary load resistor 315, coupled in parallel across the ends of across secondary winding 307.
As shown in FIG. 3, transformer 301 is a step-up type transformer, which enables the required higher level voltage signals to be achieved on data communications line 321 from a lower level voltage integrated circuit. Since transformer 301 is a step-up type, primary load resistor 309 must be a low impedance load in order for the impedance to be matched across the system. For instance, assuming step-up transformer 301 has a turns ratio of 1:1.41 and that the characteristic impedance 315 of the transformer line 321 is 100 ohms, current generator 317 must operate into an impedance of 50 ohms reflected through a 1:2 impedance transformation. This results in primary load resistor 309 being only 25 ohms in this example. This low impedance is very difficult to implement on a matched impedance circuit board layout.
A 1:1.41 ratio is chosen in this illustration because it is the lowest transformation ratio that is practical to use with 3.3 volt driver circuits. If an integral ratio, such as for example 1:2, were selected for step-up transformer 301, which is relatively easy to wind, current generator 317 would have to operate into an even lower load impedance. Specifically, if step-up transformer 301 has a turns ratio of 1:2, primary load resistor 309 would be 12.5 ohms in the case of a 100 ohm secondary load resistor 315.
Non-integral transformation ratios are difficult to achieve accurately with the low number of turns present on high frequency transformers. This is exacerbated by the fact that the output signal level, or launch level, of network drivers must be tightly controlled to allow proper operation of receive-end adaptive line equalizers. Thus, the resulting step-up transformer of FIG. 3 is difficult and expensive to manufacture and may have to be custom-matched with a lower voltage physical layer of a an integrated circuit. As integrated circuit voltages continued to decrease, such as for example to 2.2 volts or lower, correspondingly even higher step-up ratios and even lower drive impedances will need to be adopted if the configuration of the schematic shown in FIG. 3 is utilized.
Thus, what is desired is an method and an apparatus providing higher voltage output signals in communications lines from lower level voltage integrated circuits using transformers that do not suffer from the problems discussed above.
A data communications circuit generating higher voltage output signals using a lower voltage integrated circuit is disclosed. In one embodiment, a plurality of lower voltage contributory output signals are generated with a plurality of current generators. Each one of the plurality of lower voltage contributory output signals are coupled to a corresponding separate one of a plurality of primary windings of a pulse type transformer having an overall turns ratio of 1:1. The plurality of lower voltage contributory output signals in the primary windings are summed in a secondary winding of the pulse type transformer to generate the higher voltage output signal. A data communications line is coupled to the secondary winding. A composite impedance across the plurality of primary windings is matched with a characteristic impedance across the secondary winding. Additional features and benefits of the present invention will become apparent from the detailed description, figures and claims set forth below.